This invention relates generally to duct units, including sound attenuating duct units for transferring air or gases to a fan unit, including such duct units for delivering air to and from an air supply fan unit for use in conjunction with air heaters, such as those used in boiler systems.
It is known to provide large utility and industrial boilers (steam generators) which are used for power and co-generation. These boilers can be oiled fired with water tubes extending through the boiler and the water therein being heated by means of suitable air heaters. Large amounts of combustion air can be provided to these air heaters by means of a forced draft fan unit (herein sometimes referred to as a “FD fan”). This fan unit is powered electrically and can be arranged to rotate about a horizontal axis. It is known to deliver fresh air from the atmosphere through a long, vertically extending air duct that may include a splitter-type sound attenuating section. In a known air delivery system, the incoming air must change its direction through a substantial angle and must pass through a so-called trap section prior to moving horizontally into the fan inlet section.
It is also known to provide an air delivery duct that extends from the outlet of the FD fan to the bottom of an air heater unit for the boiler which can be located a substantial distance above the FD fan. The known outlet duct for the FD fan includes an elbow section in which the pressurized air flow is turned through a substantial bend typically in the order of 90 degrees. There is then an elongate duct section above the elbow section which extends upwardly and which has diverging sidewalls.
Some known difficulties or deficiencies with the duct systems for delivering air to the FD fan and for delivering the pressurized air from the fan to the air heater of the boiler include substantial power consumption for the fan, relatively high operational noise created in the vicinity of the fan unit, relatively poor air flow distribution across the air heater or air heaters and duct vibrations.
In one conventional system for an inlet duct providing combustion air to an FD fan, there is a standard splitter silencer which has an open area across the transverse cross-section of the duct between 45 and 55%. Because of the configuration of these known silencers and because air follows the path of least resistance through an air duct, the flow through the passageways formed by the splitters is not uniform. There is in fact a biased flow in the center of the splitter silencer which results in increased pressure drop across the silencer that is directly proportional to the change in the face velocity. There can also be airflow induced vibrations in the inlet duct unit due to a sudden change in the velocity of air.
With respect to the conventional form of connecting duct between the FD fan and the air heaters of the boiler, the conventional duct system does not provide a smooth flow of the high velocity air from the fan unit. The air flow from the fan can have a velocity of between 5,000 and 6,000 feet per minute and due to the sharp bend in the air duct, this air strikes the inner wall of the duct in the elbow region with substantial force. The reaction to this high velocity air creates a flow barrier and also creates very high turbulence in the transition section above the elbow section. This turbulence causes a very high pressure drop and noise in the duct. For example, a typical pressure drop in a duct having a length of 35 to 40 feet that extends between the fan and the steam coil air heater (SCAH) can be between 6 and 7 inches W.G. Also, the air flow distribution across the air heater is not uniform.
Examples of air duct silencers are found in U.S. Pat. No. 5,728,979 which issued Mar. 17, 1998 to Air Handling Engineering Ltd., these silencer units being designed for use both at the inlet end and at the outlet end of a fan unit. Each silencing apparatus has an exterior housing with an air inlet and an air outlet, one of which is connected to the fan unit. The inlet and outlet of the silencer are connected by an air flow passageway which is defined by perforated interior walls of the housing. One of these silencer units includes first and second series of splitters with the splitters in each series being spaced apart to form smaller air passageways and mounted side-by-side in a row.
In another form of outlet duct unit described in this U.S. patent, the primary passageway bends through a substantial angle of about 90 degrees from the inlet of the silencer to the outlet end. Two similar splitters are arranged in a downstream section of the silencer unit. In the bent section of the silencer unit, there are several curved splitters which also extend through a bend of about 90 degrees and one of these is a centrally located splitter.
It is an object of the present invention to provide an improved sound attenuating duct unit for delivery of air or gases to an inlet of a fan unit which unit is both good at reducing sound levels from the duct unit and fan and provides pressure drop savings.
It is a further object of the present invention to provide an improved outlet duct apparatus for connecting an outlet of an air supply fan to an air heater unit, such as one used in a boiler, this duct apparatus providing good static pressure regain and good pressure drop savings.